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Time series of large samples of instantaneous velocity fields with a 7.26δ0.99×1.18δ0.99 size in a turbulent boundary layer have been obtained by experimental measurement using time-resolved particle image velocimetry (TRPIV) with a four-camera array. The resultant experimental Reynolds number (Reτ) is 1046. The continuous spatial wavelet transform has been employed to convert the streamwise velocity fluctuations into multi-scale components in the streamwise direction at all wall-normal positions. The distribution of turbulence kinetic energy across all spatial scales and all wall-normal positions are determined from the multi-scale wavelet coefficients. The continuous temporal wavelet transform has also been utilized to resolve the temporal series of longitudinal velocity fluctuations into multi-scale temporal components at all wall-normal positions. The use of the multi-scale wavelet coefficients delivered the turbulent kinetic energy distribution across all temporal scales and all wall-normal positions. Moreover, the distribution of multi-scale flatness factors with spatial scales and normal position, before and after removal of multi-scale coherent structures, has been calculated. The turbulent structures are detected by zero-crossing the wavelet coefficients. The conditional sampling scheme and spatial phase-locked method have been employed to establish typical multi-scale structures at different wall-normal positions. The universality of the small-scale structures has been confirmed where the associated vorticity is characterized by an alternating positive and negative quadrupole along the longitudinal direction and wall-normal direction, while the streamlines can be considered a dynamic system composed of a saddle point and focal points. The scaling exponent Ϛ(p), before and after the removal of the coherent structures, has been calculated, confirming that the multi-scale coherent structures are responsible for the anomalous scaling law. [ABSTRACT FROM AUTHOR]

This paper presents experimental measurements by the time-resolved particle image velocimetry (TR-PIV) for the turbulent boundary layer (TBL) over the smooth surface, the superhydrophobic (SH) surface, and the superhydrophobic-riblets (SR) surface in an open-surface recirculating water channel. The Reynolds number based on the wall friction velocity and the thickness of the TBL over the smooth surface is 702. The SH surface and the SR surface are manufactured by the laser texturing method on the smooth surface and the riblets surface, respectively. By employing the (modified) Clauser method, a superior efficacy of the drag reduction of about 22.1% is obtained on the SR surface, while the drag reduction rate for the SH surface is about 18.7%. Comparing with the SH surface, the declining 2-order statistics in the near-wall region also indicates a significant drag reduction over the SR surface. The large-scale structure components extracted by the proper orthogonal decomposition are found to generate a majority of the Reynolds shear stress in the region y+ > 40. The strength of the large-scale features over the rough surfaces (SH and SR surfaces) at disparate wall-normal positions is visualized by the Quadrant splitting method and the conditional averaging. The appearance of the large-scale structures such as the hairpin packets characterized by the two-point correlation shows an excellent consistency with the statistics profile. The hairpin packets over the SH surface seem always smaller and weaker than those over the smooth surface. Over the SR surface, the hairpin packets in the region 0.1 ≤ y δ < 0.3 are the smallest and weakest among those over the three surfaces, while the scale and the strength of the hairpin packets exceed those over the smooth surface in the region 0.3 < y δ < 0.6. [ABSTRACT FROM AUTHOR]